The present invention relates to a method of feeding a strip of sheet material.
In particular, the invention relates to a method of feeding a continuous strip of sheet material to a user machine, for example an automatic cigarette manufacturing or packaging machine.
In conventional units for feeding strip material to a user machine, the strip is generally drawn from a roll located at a decoiling station and transferred to a device of the user machine by way of a vacuum equalizing chamber.
The equalizing chamber consists essentially of an enclosure equipped with vacuum means designed to generate a constant force by which the strip of material is attracted and caused to form a loop of variable length within the enclosure. Thus, the tension on the strip is maintained steady and equal to the vacuum force for the duration of its passage through the feed unit.
The rate at which the strip comes off the roll at the decoiling station is regulated according to the length of the loop formed by the strip inside the equalizing chamber; for example, an increase in length signifies that the strip needs to decoil more slowly, since an elongation of the loop means that the rate at which the strip is fed exceeds the rate at which the strip is being utilized by the machine.
It is therefore important in light of the foregoing, that the length of the loop formed by the strip internally of the equalizing chamber must be detected quickly and precisely; in effect, errors or delays in reading the length of the loop can easily result in significant variations in the tension on the strip, causing the material to break or affecting the quality of its use by the machine.
Moreover, a system typically able to ensure optimum control of the decoil rate will utilize PID type algorithms, which ideally require an instantaneous and continuously generated reading of the reference quantity, in this instance the length of the loop formed by the strip inside the equalizing chamber.
In the case of conventional units for feeding a strip of sheet material to a user machine, however, sensors that produce a continuous output (such as optical or ultrasound types) are not normally used inasmuch as these guarantee a correct measurement only within a predetermined and limited range of variation in the length of the loop. If the length of the strip registers outside this predetermined range as a result of abnormal operating conditions (e.g. a sudden deceleration of the user machine), the reading supplied by the continuous output type of sensor will be false and therefore the control function governing the decoil rate, which is based on the reading, becomes unreliable.
The object of the present invention is to provide a method of feeding a strip that will be simple and economical, and will also allow a quick and precise reading of the length of the loop formed by the running strip within the equalizing chamber.